Window and dialog closing improvements

Add Closed event to Window. Properly handle closing of dialogs.
A dialog is now always "closed" when Respond is called. The CloseRequested
and Closed events are fired. A respond can be canceled by a CloseRequested
handler. Added unit tests to verify all this semantic.
[Mac] Fixes for the closing event unit tests
[WPF] Window close event fixes
Fixes to match new closing event behavior for Dialog
The Imrpove WindowFrame.Close

The Close method now returns a boolean that indicates wether the window
could be closed or not.
Merge branch 'close-window-fixes'

Conflicts:
	Testing/GtkTestRunner.csproj
	Testing/WpfTestRunner.csproj